As for β-thalassemia, allogeneic HSC transplantation showed that stable mixed chimerism with donor HSC levels of 20–30% is sufficient to achieve significant hematologic and clinical improvement.9–11 Combined with the evidence that the association of SCD and HPFH results in a milder phenotype, these data predict that engraftment of >20% of HSC producing RBCs with >30% anti-sickling Hb levels could improve symptoms and reduce transfusion requirement. The gene discussed is GSTM1; the disease is Schnyder corneal dystrophy.